Reactance filters having a bandpass filter character can be constructed from a ladder/lattice-like interconnection of impedance elements and, in particular, of resonators to form so-called ladder-type or lattice arrangements. The document EP 1 196 991 A1 discloses a ladder-type filter, for example, which is constructed from acoustic single-port resonators using SAW technology (SAW=Surface Acoustic Wave). The advantage of such a reactance filter is that, by means of the number and type of the elements used, it is possible to set the filter properties and, in particular to improve the stop band suppression.
A reactance filter of ladder-type arrangement constructed from BAW resonators (BAW=Bulk Acoustic Wave) is known from document EP 1 407 546 A1, for example. In this case, BAW technology affords the advantage that it is possible to obtain frequency-accurate and power-resistant bandpass filters having a relatively low temperature response of the resonant frequency. On account of the frequency accuracy of BAW filters, the latter also finds a principal use in duplexers.
A further essential advantage of BAW technology is the high quality factors of greater than 1000 that can be achieved with BAW resonators. That allows very steep filter edges in the case of BAW reactance filters. Thus, the low temperature response and the high frequency accuracy predestine BAW filters for duplexer applications with a small separation between transmission and reception bands.
A duplexer comprises a transmission filter and a reception filter, which are usually constructed on separate substrates. Between transmission and reception bands there is often only a small band gap, which, in the case of the PCS mobile radio system used in the USA, for example, is only 20 megahertz at a center frequency of 1.9 gigahertz. Filters with G band extension even require duplexers whose band gap is reduced further to 15 megahertz. Such a small gap between transmission and reception bands of less than one percent relative width requires, besides a resonator technology having high quality factors and good temperature stability, also a filter design which enables steep filter edges for the two filters combined in the duplexer. It is particularly important that those two edges of the two passbands which face one another in terms of frequency are configured in a steep fashion in order to obtain the best possible demarcation between the two passbands. For the abovementioned G band duplexers, the necessary edge steepness taking account of manufacturing variations and temperature-dependent fluctuations is at least 5 dB/MHz. However, even steeper edges are desirable since this can increase both the specification of the filters or duplexers and the manufacturing yield in the case of a manufacturing-dictated fluctuation.
EP 1 407 546 A1, already cited, proposes steepening an edge of the passband by reducing the ratio of static and dynamic capacitance in a resonator. The antiresonant frequency of the resonator is thus increased in a targeted manner, without the resonant frequency of this resonator being changed simultaneously in this case. As a result, the coupling decreases and the passband edge assigned to the resonator becomes steeper. For the purpose of realization, it is proposed to use piezomaterials or electrode materials having lower coupling. As a further measure it is proposed to use acoustic mirrors that can likewise lower the acoustic coupling of the respective resonator.
A further possibility for configuring an edge in a steeper fashion consists in using in the reactance filter more parallel resonators that generate more pronounced or additional zeroes and therefore also steepen the edge. What is disadvantageous however, is that with this method the ohmic losses of the reactance filter also increase as a result of the number of interconnected series resonators. Furthermore, the parallel-connected resonators in the reactance filter can be connected in series with inductances, wherein zeroes can be shifted to lower frequencies. This brings about not only an increase in the isolation, but also a higher bandwidth of the bandpass filter. A steeper right-hand edge of the passband is achieved by means of an increased number of series resonators, the antiresonance of which influences the position of the edge and hence the steepness thereof.
What is disadvantageous about the known solutions for improving the edge steepness is, generally, that they either bring about higher losses or can be produced only with increased interconnection and production outlay. A further disadvantage here is that for more resonators on the chip, a larger chip area is also necessary. If two series resonators are added, implicitly at least one more parallel resonator is also required, such that a total of three additional resonators are present.